Pryor’s campaign unveiled a new website, www.cottonfacts.com , that focuses on Cotton’s support of giving the president authorization to strike Syria over a chemical weapons attack that U.S. officials say killed more than 1,400 civilians. Cotton, who was elected last year, is the only member of the state’s congressional delegation who has backed military action in Syria.

“It’s irresponsible to talk about going into Syria when so many Arkansans were opposed to it and we have so many important issues to talk about in the country and in Arkansas,” Pryor campaign manager Jeff Weaver told The Associated Press.

Weaver said he sees Syria as an issue in the campaign, despite efforts by the Obama administration to pursue a diplomatic solution to the Syrian conflict. The United States and Russia are negotiating a United Nations resolution that would require Syria to dismantle its chemical weapons stockpiles.

Cotton, an Army veteran who served in Iraq and Afghanistan, had been one of the most outspoken advocates for military action in Syria, calling it a national security issue. Cotton’s campaign on Thursday defended him and accused Pryor of running a negative campaign.

“Any day that Mark Pryor wants to compare their two very different records is a good day for Tom,” spokeswoman Caroline Rabbitt said in an email. Rabbitt also criticized the site for accusing Cotton of wanting to spend billions more in Afghanistan and Iraq.

“What’s odd about this is the material attacking tom for supporting the mission in Iraq and Afghanistan, two places where tom voluntarily served our country,” she wrote. “Surely his service earns Tom the right to an opinion, if not a fair amount of credibility, on such matters.”

Republicans widely view Pryor, who is seeking a third term, as the most vulnerable Senate incumbent running next year and repeatedly criticized the lawmaker for supporting the federal health care law in 2010. Cotton continued that criticism Thursday, questioning whether Pryor agreed with Democratic Sen. Joe Manchin of West Virginia’s call for a one-year delay in the law’s insurance mandate for individuals.

Pryor’s campaign did not immediately respond to the criticism.

The Club for Growth, which has backed Cotton’s bid to unseat Pryor, also announced Thursday it was airing radio ads around the state targeting the Democratic lawmaker. The Washington group was the first to begin running television ads focusing on Pryor earlier this year, and its political action committee endorsed Cotton the day after he announced he would challenge Pryor next year.

The 60-second spot and a website launched by the group, www.pryorforobama.com , accuses Pryor of being too closely aligned with Obama, citing his vote for the health overhaul.

“The questions keep piling up for liberal Mark Pryor,” the ad says. “Have you heard him give any answers? Probably not. Pryor wants to hide his record in Washington so he can fool voters in Arkansas.”

Pryor’s campaign called the ad proof that Cotton is backed by outside interest groups.

“These groups are bankrolling Cotton’s campaign with no concern for Arkansas, and they’re willing to spend millions to buy a Senator whose irresponsible agenda is lockstep with their own,” Weaver said.